The present invention relates to a NOx sensor detecting a concentration of NOx in a sample gas, such as an exhaust gas of an internal combustion engine, and relates to a manufacturing method of the NOx sensor.
A conventional NOx sensor, used for detecting the concentration of NOx in a sample gas, utilizes an oxygen ion conductivity of a solid electrolytic substrate (more specifically, an oxygen ion conductivity of a stabilized zirconia etc.) as disclosed in SAE 960334.
FIGS. 11 and 12 show a conventional NOx sensor 9 which comprises a reference gas chamber 950 and a sample gas chamber 902. The sample gas chamber 902 has a bottom and a ceiling defined by a pair of solid electrolytic substrates 941 and 942 each having an oxygen ion conductivity. A sensor cell 93 is provided between the reference gas chamber 950 and the sample gas chamber 902 for detecting the concentration of NOx. The sensor cell 93 has a pair of sensing electrodes 931 and 932 attached on opposite surfaces of the solid electrolytic substrate 941 located between the reference gas chamber 950 and the sample gas chamber 902. The sample gas chamber 902 communicates with another sample gas chamber 901 via a gas passage 909. A sample gas introducing passage 908 is provided adjacent to the sample gas chamber 901.
An oxygen pump cell 91 and an oxygen sensor cell 92 are provided in the sample gas chamber 901. The oxygen pump cell 91 has a pair of oxygen pumping electrodes 911 and 912 attached on opposite surfaces of the solid electrolytic substrate 942 defining the ceiling of the sample gas chamber 901. The oxygen sensor cell 92 has a pair of oxygen sensing electrodes 921 and 922 attached on opposite surfaces of the solid electrolytic substrate 941 defining the bottom of the sample gas chamber 901. The oxygen sensor cell 92 is located between the reference gas chamber 950 and the sample gas chamber 902. One oxygen sensing electrode 921 faces to the reference gas chamber 950. The other oxygen sensing electrode 922 faces to the sample gas chamber 901. The oxygen pump cell 91 and the oxygen sensor cell 92 cooperatively adjust the sample gas to have a constant oxygen concentration.
This conventional NOx sensor detects the NOx concentration in the following manner.
First, a sample gas is introduced into the sample gas chamber 901. The oxygen sensor cell 92 and the oxygen pump cell 91 cooperatively operate to adjust the oxygen gas concentration to a predetermined constant value. The sample gas having a thus adjusted oxygen gas concentration is introduced into the sample gas chamber 902 via the gas passage 909.
The sensing electrode 932 of the sensor cell 93 is an active electrode that ionizes oxygen atoms involved in a NOx gas through a deoxidizing reaction. The ionized oxygen flows across the solid electrolytic substrate 941, causing an ion current having a value proportional to an amount of the NOx gas. Thus, the concentration of the NOx gas can be detected by measuring the ion current.
On the other hand, the electrodes 911 and 922 facing to the sample gas chamber 901 are inactive electrodes having no capability of ionizing the oxygen atoms. The sensing electrode 932 (i.e., active electrode) is made of platinum (Pt). The electrodes 911 and 922 (i.e., inactive electrodes) are made of platinum (Pt) with gold (Au) additives.
Reference numeral 99 in FIG. 11 denotes terminals formed on the edge of the NOx sensor 9.
The NOx sensor 9 is manufactured in the following manner. A plurality of green sheets of a solid electrolytic substrate are prepared. Each electrode is printed on a predetermined position of a corresponding solid electrolytic substrate sheet by using an electrically conductive paste. Then, the solid electrolytic substrate sheets are stacked to form a multilayered sensor body. Finally, the multilayered sensor body is sintered at a time to finally obtain the NOx sensor 9.
However, the above-described conventional NOx sensor 9 has the following problems.
A melting point of Au, i.e., 1,064.degree. C., is so low that Au vaporizes from the electrode in a high-temperature environment when the NOx sensor is sintered according to the above-described conventional manufacturing method. The vaporized Au contaminates the active electrode. The activity of the active electrode is deteriorated by the vaporized Au. This causes a problem in the detection of the NOx concentration.
Many of NOx sensors use an oxygen ion conductive substance of a zirconia group as the solid electrolytic substrate. The sintering temperature of this substance is in a range of 1,400.about.1,600.degree. C. It means that the above-described contamination problem of the active electrode by the vaporized Au is inevitable for many of the NOx sensors.